Metal halide lamp system

ABSTRACT

A major difficulty with long linear metal halide type lamps is nonuniformity of light output along the axis of the lamp. The extent of nonuniformity increases with the number of metals included in the lamp filling and the factors primarily responsible for it are cataphoresis and thermal gradients. The cataphoretic effect is the stronger and the nonuniformity may be overcome by supplying a DC component to the discharge current and controlling its polarity and magnitude. In one system, the lamp is operated from a DC power supply through a control circuit using semi-conductors for polarity reversing switches wherein the time of current flow in one polarity is determined by the direction and degree of unbalance of the spectral output.

United States Patent Lak [451 Oct. 24, 1972 [54] METAL HALIDE LAMPSYSTEM Primary Examiner-J0hn Kominski [72] Inventor: William H. Lake,Novelty, Ohio Amway-Ems [73] Assignee: General Electric Company [57]ABSTRACT [22] Filed: Sept. 23, 1971 A major difficulty with long linearmetal halide type lamps is nonuniformity of light output along the axis[21] Appl' 183o94 of the lamp. The extent of nonuniformity increaseswith the number of metals included in the lamp filling [52] US. Cl...315/l5l, 250/205, 250/226, and the factors primarily responsible forit are 313/229, 315/155, 315/158, 323/21 cataphoresis and thermalgradients. The cataphoretic [51] Int. Cl. ..H05b 37/02 effect is thestronger and the nonuniformity may b 58 Field of Search ..315/l5l, 158,155; 250/205, Overcome y pp y a DC component to the 25002 323 21; 313discharge current and controlling its polarity and magnitude. In onesystem, the lamp is operated from a DC [56] References Cited powersupply through a control circuit using semi-conductors for polarityreversing switches wherein the UNITED STATES PATENTS time of currentflow in one polarity is determined by 2,749,501 6/1956 Bartlett..315/158 g i i and degree unbalance the spectral 3,431,427 3/1969 Pahl,Jr ..3l5/151 10 Claims, 5 Drawing Figures LUS & pus r- 9*- *EEF LLS p pELS OPERA TIONHL flMPL/F/EE POWER CON T201. AMPLIFIERS FUNCTIONAL BLOCKD/flGEflM 0F 176 8/95 CONT/POL C/ECU/ T PHENTED 24 I972 3 700 960 sum 10F 4 CONTROLLED DC Bms U) E h E L x GREEN BHND (Th'flLL um) i CONTROLLEDFig 2.3 Q DC 3/05 {5 \//-60HZ SINE WAVE S x-*- RED BHND (L/rH/L/M) 1;CONTROLLED 0 DC 3/95 E k E x BLUE BEND (IND/UM) SPECTEflL DISTRIBUTIONOFLflMP TYPE 1 lnven t'or: WiLLiam H. Lake Hi5 AtTOTTWGH PATENTED 24I97? 3. 700.960

sum 2 or 4 Fig; 3.

LUS BUS LL$ p1 p2 ELS OPE? T/0NOL HMPL/F/EE POM/2 CONTROL L 4 AMPLIFIERSFUNCTIONHL BLOC/J D/flGEflM 0F DC 8/05 CON TEOL C IECU/ T lnven tow':WiLLiam H. Lake His A t t they PATENTEDCBT 24 m2 SHEEI W 0F 4 lnventow': WiLLiam H. Lake by W I His A t tovneg BACKGROUND OF TI-IEINVENTION Long linear metal halide lamps may be designed to provideradiation in a wide variety of spectral bands as required by theparticular use. Examples of industrial applications are diazo printingrequiring peak output in the 3500 to 4000 A regiomphoto polymerizationreactions primarily sensitive in the 2000 to 2500 A region,polymerization of photo resists in the region below 2000 A,'paint dryingand air curing of coatings in the range of 2500 to 4000 A. Radiationefiiciencies of metal halide lamps specifically designed to emit inselected wavelength regions may be several times higher in those regionsthan other types of discharges.

An example of a long linear metal halide lamp of the kind in question isthat described and claimed in copending application Ser. No. 863,732,filed July 7, 1969, now abandoned by John M. Sato, William H. Lake andDelmar D. Kershaw, entitled Selective Spectral Output Metal Halide Lampand assigned to the same assignee as the present invention. One lampspecifically described therein for use in reprographic applications tomake colored copies has radiation concentrated in the three primarycolors, blue, green and red utilizes a filling of zinc, lithium andthallium iodides plus mercury.

In general, photochemical processes have photonlimited reaction ratesand some specific reactions may be inhibited by radiation in otherregions of the spectrum. This means that very high power densities inspecific spectral regions are desirable. Long linear or slender lampsfacilitate focusing of individual units as desired and also permitmounting in a closely packed array to provide very high power densities.By slender lamps are meant lamps in which the length is at least timesthe envelope diameter. However, slender discharge lamps tend to have theundesirable characteristic of nonuniformity of radiation along the axisof the lamp. U.S. Pat. No. 2,761,086 Noel and Falconer, August 1956,Electric Discharge Lamp, provided a solution to this problem for thecase of high pressure mercury'vapor lamps emitting at 3650 A. Theproblem of nonuniformity is very much aggravated with metal halide lampscontaining more than one metal. In such lamps the spectral outputdistribution becomes nonuniform along the axis, that is the color variesdrastically along the length of the lamp. Unless the nonuniformity canbe eliminated, the lamp is useless for reprographic applications. Thedegree of nonuniformity depends upon the particular metal halides usedand limits the number and permissible amount of suitable materials. Inaddition arc chamber parameters must be carefully controlled in order toassure that lamps have a uniform spectral distribution even foracceptable metal halides. In all cases, a practical limitation exists onthe length of the discharge which can be achieved without developingspectral nonuniformity.

By way of example, metal halide discharges in T3 (tubular inch arc tubeslonger than 4 inches show in general at least 10 percent variation inspectral output from end to end and often as much as 100 percent whenoperated on alternating current. When the same lamps are operated ondirect current, even greater nonuniformity results with vapor speciesbeing pumped to one end or the other of the tube by electrophoresis.

The direction and degree of pumping varies with the particular metalhalide species and with the other metals present in the arc tube.

SUMMARY OF THE INVENTION 1 have found that the factors primarilyresponsible for nonuniformity are thermal gradients and cataphoresis. InAC discharges thermal gradients may be caused by a non-uniform bulbtemperature distribution, or be variations in bulb diameter, or byunbalanced electrode losses. Regenerative conditions may exist causingsmall unbalances to lead to large nonuniforrnities. Cataphoresis mayresult directly from the nature of the current used, for instance wherethe current is DC or where the current is AC which includes a DCcomponent. Cataphoresis may also result from an unbalance in theelectrode energy input which effectively creates a DC component.

I have found that the electrophoretic effects resulting from theintroduction of a direct component into the lamp current are fast actingand relatively powerful. By introducing a controlled DC component intothe lamp operating current, the effects of the relatively weak phenomenadue to thermal gradients may be overridden. The nonuniformity may beregulated and effectively overcome by controlling the direction andmagnitude of the DC component introduced into the discharge current.

There are various ways in which the DC component may be supplied andcontrolled. One way, which may be referred to as Variable Time RatioSquare Wave Operation, operates the lamp from a DC power supply throughpolarity reversing switches in the form of semiconductors. A controlcircuit is provided whereby the time of current flow in any one polarityis determined by the direction and degree of unbalance in the spectraloutput.

In another circuit which may be referred to Audio Frequency with DCOffset Current, the lamp is operated directly from a high audiofrequency power supply. An auxiliary control provides a parallel DCoffset current to regulate the electro-phoretic effect in the lamp.

DESCRIPTION OF DRAWINGS FIG. 1 illustrates a long linear tubular metalhalide lamp suitable for the invention.

FIG. 2 compares the output distribution of the lamp of FIG. 1 undernormal 60 hertz operation with that under controlled DC bias for threecolor bands.

FIG. 3 is a functional block diagram of a DC bias control circuit foroperating the lamp according to the invention.

FIG. 4 shows waveform output of an operational amplifier in the controlcircuit.

FIG. 5 is a detailed schematic diagram of the DC bias control circuit.

DETAILED DESCRIPTION Lamp Structure A slender tubular lamp suitable foroperation according to the invention is illustrated in FIG. 1 comprisingan arc tube 1 of quartz or fused silica. The tube may be of the sizecommonly referred to as T3 (tubular, it: inch dia.) having an insidediameter of about 8 millimeters and having sealed therein at oppositeends a pair of arcing electrodes 2,2 defining an arc gap of about 14inches. In the drawing, portions of the lamp have been cut away toshorten the figure. The electrode inleads 3 have intermediate thinmolybdenum foil sections 4 hermetically sealed through full diameterpinch seals 5 at the ends of the tube. The electrodes each comprise adouble layer tungsten wire helix 6 wrapped around a tungsten core wire7, and may be conventionally activated by thoriumoxide applied as acoating on the turns of the helix or filling the space or intersticesbetween turns. In order to heat the. end chambers and thus increase thevapor pressure of the metal iodides, quartz sleeves 8 extendingforwardup to the electrode tips may be fitted over the ends of the lamp asillustrated. The space between the tube wall and the sleeve is filledwith a refractory white insulation 9 of quartz fibers.

EXAMPLE 1 In one set of lamps utilizing the above-described envelope,the filling consisted of the following:

Thallium Iodide 10 mg Lithium Iodide 5 mg Indium Iodide l0 mg Mercury 7mg Argon torr Green 50004500 A Red 6100-6800 A Blue Under conventionalsine wave alternating current operation, it was possible to maintainspectral uniformity with the above lamp filling in arc lengths up toabout 10 inches. However this required close control of the arc chamberdiameter, namely limiting variations in diameter to less than 10 micronsper centimeter of length, close control of wall thickness, and closeregulation of end chamber temperature. When the same lamps were operatedutilizing a DC bias control circuit in accordance with the invention,satisfactory spectral uniformity was achieved for 14 inches are lengthlamps even when no attempt was made to control the important arc chamberparameters. FIG. 2 shows the output distribution for normal 60 hertzoperation (dashed lines), and for operation with a controlled DC bias(solid lines) according to the invention, for the three color bandsgreen, red and blue. In each case the intensity is plotted as ordinateagainst the place of measurement along the length of the lamp asabscissa, and the improvement in uniformity is apparent.

EXAMPLE 2 In a second lamp type utilizing an envelope similar to thatpreviously described, the filling consisted of the following:

Thallium Iodide 10 mg Lithium Iodide l0 mg zinc iodide 10 mg Mercury 7mg Argon 25 torr 14 inches.

Operation with DC bias control permits the use of a greater variety ofmetal halides with more flexibility in the choice of spectral output. Itreduces the need for close control of arc chamber parameters, therebylowering the cost of manufacture and the shrinkage rate. It permits theuse of very small chamberdiameters in linear lamps, for example inapplications requiring critical focusing.

BLOCK DIAGRAM The block diagram of FIG. 3 illustrates one scheme forcontrolling a DC bias current to regulate the spectral distribution of ametal halide discharge lamp. A

pair of photo sensors P1 and P2 responsive to light intensity are aimedat opposite ends of the lamp. Their signals are fed to a differentialoperational amplifier connected as a zero crossing detector. The outputvoltage of the operational amplifier is supplied to power controlamplifiers which provide an on signal to the right or left uppertransistor power switches (LUS or RUS), and to the diagonally oppositelower transistor power switch (RLS or LLS). The four transistor powerswitches determine the direction in which the DC supply current isallowed to flow through the lamp load.

Circuit Description A circuit operating in accordance with the foregoingscheme is shown schematically in FIG. 4. Photosensors P1 and P2 comprisephoto transistors Q1 and Q2 aimed or optically coupled to receiveradiation f ,f,, from opposite ends of the lamp 1. They determine theinput voltage to the darlington photo amplifiers, 03,4 and Q5 ,0. Tocompensate for variation between the response of individual phototransistors to light flux, a sensitivity adjustment of the gain andoutput voltage level of the photo amplifiers is made by means ofvariable resistors R1 and R2 (of 50 kilohms) and R3 and R4 of 5 kilohms.The sensitivityadjustment also determines the overall response of thesystem to an imbalance of light at the lamp ends. It is adjusted toprovide sufficiently fine control to meet the need for uniformity butnot so high as to cause hunting or oscillation of the end to end lightdifference.

The output of the photo amplifiers is combined with an alternating sinewave voltage from source AC and applied to the input terminals x and yof a conventional operational amplifier 0A connected as 'a zero crossingdetector. If the input at terminal at exceeds that at terminal y, theoutput is negative;.if the input at terminal y exceeds that at terminal1:, the output is positive.

The control function is developed as follows. The input signal to theoperational amplifier is the difference in voltage applied to terminalsx and y and is given by:

V, V,,= Vsin wt+(V V where V sin wt is the alternating sine wave voltagefrom source AC, V, is the signal at x-due to sensor P1, and V is thesignal at y'due to sensor P2. By design, the ac sine wave voltage isalways greater than the difference in sensor voltages V V Then when V Vthe input voltage to 0A is as shown in FIG. 4a. When the input to 0A ispositive, the output is negative, as shown in FIG. 4b and vice versa.When V V then OA simply performs a squaring function on the input sinewave, as shown in FIG. 40. When V V the input voltage is as shown inFIG. 4d, and the output voltage as shown in FIG. 4e.

The-average output voltage V out of operational am- 'where T= l/w periodof the input sine wave, V is the peak of the sine wave, At, is thepositive wave duration, and At is the negative wave duration. Its valuecan vary continuously from +V to V,, depending on the difference in thelight output between the lamp ends.

The control voltage must operate to balance the light output fromopposite ends of the lamp. For the lamp of Example 2 above, a dc voltageapplied to the lamp will cause the light to increase at the cathode ornegative end. The control circuitry must provide current through thelamp in a direction to counteract the sensed imbalance 'by reversing thedccomponent of current.

Referring to FIG. 5, a bridge circuit controls current flow from aconstant current dc source 11. The bridge comprises four transistorpower switches LUS, RUS, LLS and RLS, being left and right, upper andlower switches. The polarity of power application is determined byhaving LUS and RLS on at the same time as RUS and LLS are off, and viceversa. For reliability, neither LUS andRLS, nor LLS and RLS should everbe on simultaneously, and it is desirable that the transition fromone'polarity to, the other be made as rapidly as possible. Theserequirements are fulfilled by the drive circuitry intervening betweenoperational amplifier 0A and the transistor power switches.

Drive Circuitry Transistor 07 is a very high gain transistor and servesto increase the level of the output from operational amplifier CA aswell as to speed up the voltage reversal rate to approximately 20,000volt/sec. Transistor Q8 inverts the output of Q7, that is reverses itsphase. Transistors 07 and Q8 thus provide the out of phase on and offcontrol signals necessary to obtain the proper application of power tothe lamp through the power switches.

Transistors 09,10 and 011,12 provide high peak power capacity fordriving the principal drive transistors 013,14 and 015,16 as rapidly aspossible. The associated circuitry provides isolated control of the tumon and turn off signals to maximize the speed and at the same timemaintaining a very low dissipation. This permits lower cost low powertransistors to be used for the drive transistors.

6 In the absence of any drive sigials, all the power switches are biasedoff by the passive bias supplies. The

upper power switches LUS and RUS have individual low voltage biassupplies consisting of the full wave rectified output of transformers Tand T and the bias control transistors 017 and 018. The lower powerswitches LLS and RLS operate from a single bias supply (not shown) withsimilar circuitry connected to terminals 12 and 13. These bias suppliesinsure that the power switches are in saturation at the maximum currentlevel required for the lamp.

The principal drive transistors are connected to the base circuit of thebias control transistors 017,18 and 019,20 and when on remove the basedrive from the power switch circuits. A problem with such circuitryarises from the fact that transistors turn on much more rapidly than 0and turn off from saturation can be even longer due to stored charge inthe transistors (storage time). Since LUS and LLS must never be onsimultaneously (similarly RUS and RLS), the turn on of these transistorsmust. be made slower than the turn off including storage time, thereverse of the inherent tendency. While delay networks can be used, theybecome complicated and result in high dissipation in the power switcheswhen long time periods are involved. In particular, the darlingtonconnection .results in much longer charge storage and slower turn OKbecause the conventional means of charge removal consisting in reversebiasing the emitter-base junction, cannot be used. According to animportant feature of the invention, this handicap is overcome by meansof pulldown diodes.

The diodes D1, D2-5; D6, D740; D11, D12-15; and D16, D17-20 providepaths for removal of stored charge from each of the transistors throughthe principal drive transistors and thus serve to minimize the storagetime. The improvement in performance is remarkable: typical storage timewithout these diodes is 20 microseconds or more; with the pulldowndiodes, it is on the order of 2.5 microseconds.

1 The addition of very small capacitors Cl,2,3,4; delays the turn ontime up to about 3 microseconds without seriously affecting the risetime and increasing thedissipation. The diode strings D2l-24 and D25-28provide a convenient means of establishing a negative turn off bias formaximizing the turn off rate of the lower transistors. The uppertransistors do not need this because they turn off into the +DC powersource which provides ample pulldown drive. Lamp-Circuit FunctionalConsiderations The reignition voltage of the long linear metal halidelamps used with this system can be extremely high. For 60 hz sine waveoperation, reignition voltages greater than 2000 volts are oftenobserved during the starting period, and greater than 1000 volts duringnormal operation. High reignition voltages make ballasting difficult.However with dc or square wave operation as provided by the invention,the problem is minimized. In square wave operation, the transition mustbe sufiiciently rapid to limit the reignition voltage to the capa'bility of the dc power source. Transition times of the order of 20-30microseconds can result in reignition voltages of -200 volts and cancause difficulty in the design of the power source. Therefore it isdesirable to make the transition time less than 20 microseconds,

and transition times on the order of 2 to l0.

microseconds, as achieved by this circuit, are much preferable. Thusavoidance of high reignition voltages is another advantage of thecircuit according to the in vention.

What I claim as new and desire Patent of the US. is:'-

1. A circuit for overcoming nonuniformity of radiant output along thelength of an elongated metal vapor discharge lamp of a kind subject toelectrophoretic effects, comprising:

a source of electric current and means for connecting the lamp acrosssaid source,

sensing means responsive to unbalance in spectral output between theends of said lamp,

and control means responsive to said sensing means and providing currentthrough said lamp with a dc component proportioned to create anelectrophoretic effect overcoming said unbalance.

2. A lamp as in claim 1 wherein said control means providescyclicreversals of current through said lamp in addition to said dc component.

to secure by Letters 3. A circuit as in claim 1 wherein said sensingmeans 7 comprises a pair of radiation sensors optically coupled toopposite ends of said lamp.

4. A circuit as in claim 1 wherein said source provides a controlled dccurrent and said means for connecting the lamp across said sourcecomprises semiconductor switches controlled by said control means.

5. A circuit as in claim 1 wherein said source provides a controlled dccurrentand said means for connecting the lamp across said sourcecomprises a bridge circuit including four semi-conductor switchescontrolled by said control means in pairs to determine the direction ofcurrent flow through said lamps.

6. A circuit as in claim 1 wherein said sensing means comprises a pairof radiation sensors optically coupled to opposite ends of said lamp,said source provides a controlled dc current, and said means forconnecting the lamp across said source comprises a bridge circuitincluding four semiconductor switches controlled by said control meansin pairs to determine the direction of current flow through said lamps.

7. A circuit for overcoming nonuniformity of radiant output along thelength of an elongated metal vapor 8. lamp of a kind subject toelectrophoretic effects, comprising:

a source of controlled electric dc current,

I a bridge circuit comprising four semiconductor switches, 7 said sourcebeing connected across one diagonal of said bridge,- 7 terminals forconnecting said lamp across the other diagonal of said bridg a pair ofradiation sensors optically coupled to opposite ends of said lamp andresponsive to the unbalance in spectral output therefrom,

control means cyclically turning on" and off opposite pairs of saidswitches in order to provide alternating current flowfrom said sourcethrough said lamp,

said control means being responsive to the signals from said sensors byvarying the time of current flow in any one polarity in accordance withthe direction and degree of unbalance in the spectral 8. lil iic itit asin claim 7 wherein said control means comprises an ac source and anoperational amplifier, said operational amplifier providing a squarewave output reversing at the frequency of said ac source with a ratio oftime duration in opposite polarities following I the degree of unbalancein the spectral output.

9. A circuit as in claim 7 wherein each of the four switches comprisestwo transistors connected emitter to base with common collectors in thedarlington mode, and diodes connecting the bases of said transistors tosaid control means in order to achieve fast removal of stored carriersat turn off.

10. A circuit as in claim 7 wherein said control means comprises an acsource and an operational amplifier, said operational amplifierproviding a square wave output reversing at the frequency of said acsource with a ratio of time duration in opposite polarities followingthe degree of unbalance in the spectral output, and each of the fourswitches comprises two transistors connected emitter to base with commoncollectors in the darlington mode, and diodes connecting the bases ofsaid transistors to said control means in order to achieve fast removalof stored carriers at turn off.

UNITED STATES PATENT orrmr QERTEFEQATE M CQRREQTEGN Patent NO. 3,700,960Dated October 24, 1972 Inventor(s) WILLIAM H. LAKE It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 1, line 20, after "abandoned", insert in favor ofcontiLnuation-in-part application Serial No.

218,491, filed January 17, 1972,

Signed and sealed this 20th fiay of November 1973.

(SEAL) Attest 2 EDWARD Ni.FlZ-FIICHER ,3B5.a RENE D. 'JE'EGTMEYERAttesting Officer Acting Commissioner of Patents FORM P0-1050 (10-69)UscOMM. 375.p69

# U.S. GQVERNMENT PRlNTING OFFICE: I989 O-366-334.

1. A circuit for overcoming nonuniformity of radiant output along thelength of an elongated metal vapor discharge lamp of a kind subject toelectrophoretic effects, comprising: a source of electric current andmeans for connecting the lamp across said source, sensing meansresponsive to unbalance in spectral output between the ends of saidlamp, and control means responsive to said sensing means and providingcurrent through said lamp with a dc component proportioned to create anelectrophoretic effect overcoming said unbalance.
 2. A lamp as in claim1 wherein said control means provides cyclic reversals of currentthrough said lamp in addition to said dc component.
 3. A circuit as inclaim 1 wherein said sensing means comprises a pair of radiation sensorsoptically coupled to opposite ends of said lamp.
 4. A circuit as inclaim 1 wherein said source provides a controlled dc current and saidmeans for connecting the lamp across said source comprises semiconductorswitches controlled by said control means.
 5. A circuit as in claim 1wherein said source provides a controlled dc current and said means forconnecting the lamp across said source comprises a bridge circuitincluding four semi-conductor switches controlled by said control meansin pairs to determine the direction of current flow through said lamps.6. A circuit as in claim 1 wherein said sensing means comprises a pairof radiation sensors optically coupled to opposite ends of said lamp,said source provides a controlled dc current, and said means forconnecting the lamp across said source comprises a bridge circuitincluding four semiconductor switches controlled by said control meansin pairs to determine the direction of current flow through said lamps.7. A circuit for overcoming nonuniformity of radiant output along thelength of an elongated metal vapor lamp of a kind subject toelectrophoretic effects, comprising: a source of controlled electric dccurrent, a bridge circuit comprising four semiconductor switches, saidsource being connected across one diagonal of said bridge, terminals forconnecting said lamp across the other diagonal of said bridge, a pair ofradiation sensors optically coupled to opposite ends of said lamp andresponsive to the unbalance in spectral output therefrom, control meanscyclically turning ''''on'''' and ''''off'''' opposite pairs of saidswitches in order to provide alternating current flow from said sourcethrough said lamp, said control means being responsive to the signalsfrom said sensors by varying the time of current flow in any onepolarity in accordance with the direction and degree of unbalance in thespectral output.
 8. A circuit as in claim 7 wherein said control meanscomprises an ac source and an operational amplifier, said operationalamplifier providing a square wave output reversing at the frequency ofsaid ac source with a ratio of time duration in opposite polaritiesfollowing the degree of unbalance in the spectral output.
 9. A circuitas in claim 7 wherein each of the four switches comprises twotransistors connected emitter to base with common collectors in thedarlington mode, and diodes connecting the bases of said transistors tosaid control means in order to achieve fast removal of stored carriersat turn off.
 10. A circuit as in claim 7 wherein said control meanscomprises an ac source and an operational amplifier, said operationalamplifier providing a square wave output reversing at the frequency ofsaid ac source with a ratio of time duration in opposite polaritiesfollowing the degree of unbalance in the spectral output, and each ofthe four switches comprises two transistors connected emitter to basewith common collectors in the darlington mode, and diodes connecting thebases of said transistors to said control means in order to achieve fastremoval of stored carriers at turn off.